Such a device comprises a stir pawn designed to pass through the material to be welded and to be stirred as the welding device is advanced, as well as a lower shoulder and an upper shoulder. The lower shoulder is mounted fixed on the stir pawn, while the upper shoulder is mounted in translation along the stir pawn. It is thus possible to have a variable spacing between the two shoulders, between which the parts to be welded are intended to be housed. The double-shouldered welding device makes it possible to create a heating of the two sides of the welded joint and hence to have a better heat distribution. In addition, the lower shoulder absorbs the pressure force exerted by the upper shoulder, which makes it possible to weld parts that are not capable of absorbing the forging force of a conventional tool or a retractable pawn.
Currently, there are two major modes of controlling a double-shouldered welding device for the friction stir welding of parts, i.e., a controlled force control and a controlled spacing control between the two shoulders.
Controlled force or forced control is easier to use. It consists of applying a constant force between the two shoulders of the welding device. However, such a use does not make it possible to have a welded joint of constant thickness and may lead to significant decreases in thickness, or even to collapses of the joint. In fact, if the working temperature between the shoulders increases during the welding method, the forced control tends to cause an excessive compression of the material to be welded, which is accompanied by a discharge of the plasticized material to the outside of the welding device. In fact, with the resistance of the material decreasing more and more with the rise in temperature, maintenance of a constant force tends to require a greater and greater compression by the shoulders, which causes a racing of the system. Such a method is hence difficult to control.
Conversely, an applied spacing control makes it possible to work in a dimensional manner, to control the thickness of the welded joint and thus to obtain a joint of higher quality. Spacing is defined as the distance between the two shoulders of the welding device. Compression is defined as the difference between the thickness of the parts to be welded and that of the welded joint, i.e., the difference between the thickness of the parts to be welded and the spacing. Recent studies have made it possible to show that for sheets of a few mm, a very high quality is obtained for the welded joint by requiring a compression on the order of 100 μm. However, by fixing the spacing between the shoulders, the welding device may be led to require variable (too high or insufficient) forces on the parts to be welded because such a control mode cannot take into account slight variations in thickness of the parts to be welded. It necessitates means for measuring the thickness variations to take into account these variations and to allow a change in the spacing between the two shoulders.